Methods for producing aluminum trifluoride

ABSTRACT

Methods for producing aluminum trifluoride by acid digestion of fluoride salts of alkali metal or alkaline earth metal and aluminum, optionally, in the presence of a source of silicon; methods for producing silane that include acid digestion of by-products of silane production to produce aluminum trifluoride.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/291,141, filed Dec. 30, 2009, which is incorporated herein byreference in its entirety.

BACKGROUND

The present disclosure relates to methods for producing fluoridecompounds and, particularly, methods for producing aluminum trifluorideby acid digestion of fluoride salts of alkali metal or alkaline earthmetal and aluminum.

Silane is a versatile compound that has many industrial uses. In thesemiconductor industry, silane may be utilized for deposition of anepitaxial silicon layer on semiconductor wafers and for production ofpolycrystalline silicon. Polycrystalline silicon is a vital raw materialused to produce many commercial products including, for example,integrated circuits and photovoltaic (i.e., solar) cells that may beproduced by thermal decomposition of silane onto silicon particles in afluidized bed reactor.

Silane may be produced by reacting silicon tetrafluoride with an alkalior alkaline earth metal aluminum hydride such as sodium aluminumtetrahydride as disclosed in U.S. Pat. No. 4,632,816 which isincorporated herein by reference for all relevant and consistentpurposes. Production of silane may result in several by-products such asvarious fluoride salts of alkali metal or alkaline earth metal andaluminum (e.g., NaAlF₄, Na₅Al₃F₁₄ and Na₃AlF₆). Conventionally, thesewaste products are sold at low prices or are disposed of in a landfill.

Aluminum trifluoride is a versatile material that may be used as acomponent in an electrolyte melt for production of aluminum and may beused in various fluorination reactions. Aluminum trifluoride isconventionally produced by reacting hydrogen fluoride with relativelyexpensive alumina or alumina trihydrate. Silicon tetrafluoride is also aversatile material that may be used to produce silane or varioushalosilanes and can be used for ion implantation, plasma deposition offluorinated silica, production of pure silica or of silicon nitride andmay be used as a metal silicide etch.

A continuing need exists for methods to reuse the wastes produced duringsilane production to reduce the amount of material that must belandfilled or cheaply sold and to improve the economics of producingsilane and resulting commercial products (e.g., photovoltaic cells). Aneed also exists for methods for producing valuable raw materials suchas aluminum trifluoride and silicon tetrafluoride.

SUMMARY

In one aspect of the present disclosure, a method for producing aluminumtrifluoride includes contacting a fluoroaluminate feed with an acid toproduce aluminum trifluoride and at least one by-product. The feedcontains at least about 30% by weight fluoride salts of alkali metal oralkaline earth-metal and aluminum.

In another aspect, a method for producing aluminum trifluoride includescontacting a fluoride salt of alkali metal or alkaline earth-metal andaluminum with an acid to produce aluminum trifluoride and at least oneby-product. The aluminum trifluoride is separated from the by-product.

Yet a further aspect of the present disclosure is directed to a methodfor producing silane and aluminum trifluoride. The method includescontacting silicon tetrafluoride and an alkali or alkaline earth-metalsalt of aluminum tetrahydride to produce silane and an effluent. Theeffluent contains a fluoride salt of alkali metal or alkalineearth-metal and aluminum. The effluent is contacted with an acid toproduce aluminum trifluoride and at least one by-product. The aluminumtrifluoride is separated from the by-product.

Various refinements exist of the features noted in relation to theabove-mentioned aspects of the present disclosure. Further features mayalso be incorporated in the above-mentioned aspects of the presentdisclosure as well. These refinements and additional features may existindividually or in any combination. For instance, various featuresdiscussed below in relation to any of the illustrated embodiments of thepresent disclosure may be incorporated into any of the above-describedaspects of the present disclosure, alone or in any combination.

DETAILED DESCRIPTION

Provisions of the present disclosure include methods for producingfluorides (e.g., aluminum trifluoride or silicon tetrafluoride) bydigestion of fluoride salts of alkali metal or alkaline earth metal andaluminum. The digestion reaction may occur in an aqueous environment orin a substantially anhydrous environment. Other provisions includemethods for producing silane and fluoroaluminate by-products and use ofsuch by-products for production of a raw material selected from aluminumtrifluoride and silicon tetrafluoride.

Generally, the reaction proceeds by contacting a fluoride salt of alkalimetal or alkaline earth metal and aluminum with an acid selected fromsulfuric acid and hydrochloric acid to produce a fluoride compound(e.g., aluminum trifluoride or silicon tetrafluoride) and variousby-products such as hydrogen fluoride and a chloride or sulfate salt ofan alkali or alkaline earth-metal. The reaction may proceed in thepresence of a source of silicon in which case silicon tetrafluoride isproduced. If the reaction occurs in the absence of a source of silicon,aluminum trifluoride is produced.

For purposes of the present disclosure, “fluoride salts of alkali metalor alkaline earth metal and aluminum” include compounds of the generalformula M_(x)Al_(y)F_(z), where x, y and z are integers from 1 to 20 oreven from 1 to 10 and M is an alkali metal or alkaline earth metal. Thefluoride salts may also generally be referred to as “fluoride aluminumsalts,” “fluoroaluminates” or simply “salts” without departing from thescope of the present disclosure. Generally, the structure of the salt isnot essential to the present disclosure and any salts that contain afluorine atom, aluminum atom and an atom of alkali or alkaline earthmetal may be used without limitation. In some embodiments, the fluoridesalt used in accordance with the present disclosure include compounds ofthe general formula M_(x)Al_(y)F_((2x/p+3y)), where M is an alkali oralkaline earth-metal and p is 2 when M is an alkali and p is 1 when M isan alkaline earth-metal.

Without being bound to any particular theory, it is believed that thereaction that occurs when a fluoroaluminate and hydrochloric acid arecontacted in the absence of silicon may be represented by the followinggeneric formula,M_(x)Al_(y)F_((2x/p+3y))+(2x/p)HCl→yAlF₃+(2x/p)HF+xMCl_(2/p)  (i),wherein M is an alkali or alkaline earth-metal and p is 2 when M is analkali and p is 1 when M is an alkaline earth-metal. For instance, whenthe fluoride salt of aluminum is NaAlF₄, the reaction proceeds asfollows,NaAlF₄+HCl→AlF₃+HF+NaCl  (ii).

When the salt is Na₅Al₃F₁₄ (also known as chiolite), the reactionproceeds according to reaction (iii),Na₅Al₃F₁₄+5HCl→3AlF₃+5HF+5NaCl  (iii).

When the salt is Na₃AlF₆ (also known as cryolite), the reaction proceedsaccording to reaction (iv),Na₃AlF₆+3HCl→AlF₃+3HF+3NaCl  (iv).

When the salt is Ba₃Al₂F₁₂, the reaction proceeds according to reaction(v),Ba₃Al₂F₁₂+6HCl→2AlF₃+6HF+3BaCl₂  (v).

When the fluoroaluminate is contacted with the acid in the presence of asource of silicon (such as SiO₂), it is believed the reaction proceedsas follows,M_(x)Al_(y)F_((2x/p+3y))+(x/2p+3y/4)SiO₂+(2x/p+3y)HCl→(x/2p+3y/4)SiF₄+(x/p+3y/2)H₂O+xMCl_(2/p)+yAlCl₃  (vi),wherein M and p are defined as above. For instance, when the fluoridesalt of aluminum is NaAlF₄, the reaction proceeds according to reaction(vii),NaAlF₄+SiO₂+4HCl→SiF₄+2H₂O+NaCl+AlCl₃  (vii).

When the salt is Na₅Al₃F₁₄, the reaction proceeds according to reaction(viii),Na₅Al₃F₁₄+3.5SiO₂+14HCl→3.5SiF₄+7H₂O+5NaCl+3AlCl₃  (viii).

When the salt is Na₃AlF₆, the reaction proceeds according to reaction(ix),Na₃AlF₆+1.5SiO₂+6HCl→1.5SiF₄+3H₂O+3NaCl+AlCl₃  (ix).

When the salt is Ba₃Al₂F₁₂, the reaction proceeds according to reaction(x),Ba₃Al₂F₁₂+3SiO₂+12HCl→13SiF₄+6H₂O+3NaCl+2AlCl₃  (x).

While the reactions above are shown using HCl as a starting material, itshould be understood that other acids such as sulfuric acid may be usedwithout limitation. In this regard, it is to be noted that the abovereactions are made only for the purposes of illustration and should notbe viewed in a limiting sense.

An exemplary embodiment of the methods of the present disclosureincludes introducing a fluoroaluminate and an acid (e.g., HCl orsulfuric acid) into a reaction vessel optionally with or without asource of silicon. A fluoride product such as aluminum trifluoride(AlF₃) or silicon tetrafluoride (SiF₄) and several by-products areproduced. The fluoride product and the by-products and any unreactedstarting materials may be introduced into a purification system toseparate the fluoride product and/or purify and isolate by-products.

Reaction Starting Materials

In various embodiments, the fluoroaluminate feed material (synonymously“fluoroaluminate feed,” “fluoroaluminate effluent” or simply “effluent”)includes an alkali metal or alkaline earth-metal fluoroaluminate.Suitable alkali or alkaline earth-metal fluoroaluminates include lithiumfluoroaluminates, sodium fluoroaluminates, potassium fluoroaluminates,magnesium fluoroaluminates, barium fluoroaluminates, calciumfluoroaluminates and mixtures thereof. In view of the wide availabilityof sodium feedstocks, such as caustic soda and potash, that may beeconomically reacted to produce sodium aluminum hydride, which may bereacted with silicon tetrafluoride to produce silane, thefluoroaluminate may be a sodium fluoroaluminate produced as a by-productof silane production. More than one fluoraluminate may be included inthe fluoroaluminate feed without departing from the scope of the presentdisclosure. The fluoroaluminate feed may include at least one of NaAlF₄,Na₅Al₃F₁₄, and Na₃AlF₆, and, in some embodiments, includes a mixture ofNaAlF₄, Na₅Al₃F₁₄ and Na₃AlF₆.

The purity of the fluoroaluminate feed is not critically important asunreacted impurities in the feed may be removed during subsequentprocessing. The fluoroaluminate feed may include an amount of silicontrifluoride, alkali or alkaline earth metal fluoride and/or chloridesalts of alkali or alkaline earth-metals and/or aluminum or otherimpurities. In various embodiments, the fluoroaluminate feed containsless than about 15% by weight impurities on a dry basis or even lessthan 10% by weight impurities. For purposes of the present disclosure,the term “impurities” refers to compounds other than fluoroaluminatessuch as, for example, aluminum trifluoride and fluoride salt (e.g.,NaF).

The amount of moisture in the fluoroaluminate feed is not critical.Generally, the fluoroaluminate feed may be solid and/or dry (i.e.,generally flowable); however, in some embodiments the fluoroaluminatefeed is dissolved in a solvent. Generally, if a solvent is used, asolvent other than water is preferred due to low solubility offluoroaluminates in water. Suitable solvents may be non-polar andinclude, for example, dimethoxyethane (DME) and toluene. Solidfluoroaluminate feed may contain less than about 5%, less than about 1%or even less than about 0.1% by weight water. The particle size of thefluoroaluminate feed may be relatively small to facilitate solidsreactivity; however, the feed material should be sufficiently large toallow the material to be handled without significant difficulty. In oneor more embodiments, the particle sizes of the fluoroaluminate feed maybe less than about 500 μm and, in other embodiments, less than about 300μm, from about 100 μm to about 500 μm or from about 200 μm to about 300μm. In some embodiments, fluoroaluminates are included in an aqueoussolution for transport of the material to the reaction vessel (i.e., asluice-type system may be utilized).

The fluoroaluminate feed may be produced by any of the known methods forproducing a fluoroaluminate (or fluoroaluminates if more than one areused) including processes wherein a fluoroaluminate is produced as aby-product. In some embodiments, the fluoroaluminate feed is aby-product of silane production. Silane may be produced by reacting analuminum hydride (e.g., lithium or sodium aluminum tetrahydride) withsilicon tetrafluoride as described below under the section entitled“Production of Silane and Fluoride Product” and in U.S. Pat. No.4,632,816, which is incorporated herein by reference for all relevantand consistent purposes. Generally, such processes produce a liquidreaction medium with by-product solids (dissolved or slurried) includedin the reaction medium. The by-product solids typically include a largeamount of fluoroaluminates and may be used as the fluoroaluminate feedof the present disclosure.

The amount of fluoroaluminates in the fluoroaluminate feed may be atleast about 30% by weight of the fluoroaluminate feed on a dry basisand, in other embodiments, is at least about 50%, at least about 70%, atleast about 80%, at least about 90%, from about 30% to about 95% or fromabout 70% to about 95% by weight fluoroaluminates by weight of the feedon a dry basis.

Generally, the fluoroaluminate feed is reacted with an acid present inan acid feed stream as more fully described below. Suitable acidsinclude HCl, sulfuric acid or a mixture thereof In certain embodiments,the acid feed stream contains HCl and may contain HCl as the only acidpresent in the acid feed stream. In embodiments wherein HCl is includedin an aqueous solution, the concentration of HCl may be, on a weightbasis, at least about 2.5%, at least about 7.5%, at least about 9%, fromabout 3% to about 20% or from about 3% to about 15% of the aqueoussolution. In embodiments wherein sulfuric acid is included in an aqueoussolution, the concentration of sulfuric acid may be, on a weight basis,at least about 50%, at least about 75%, at least about 90% or from about75% to about 99% of the aqueous solution.

A mixture of sulfuric acid and HCl may be used in the acid feed stream.The mixture may contain at least about 10% HCl by weight on a dry basis,at least about 25%, at least about 50%, at least about 75% or even atleast about 90% HCl by weight on a dry basis. In certain embodiments,the acid feed contains HCl and not sulfuric acid or may contain sulfuricacid and not HCl.

In other embodiments, the acid is a substantially anhydrous gas stream.“Substantially anhydrous” for the purposes of the present disclosuregenerally refers to process streams that contain less than about 5% byweight water. In some embodiments, the acid feed contains less thanabout 1% by weight water or even less than about 0.1% by weight water.

As noted above, a source of silicon may optionally be included in thereaction mixture. The presence of silicon determines the fluorideproduct (i.e., SiF₄ forms in the presence of silicon while AlF₃ forms inits absence). Sources of silicon include sand (i.e., SiO₂), quartz,flint, diatomite, mineral silicates, metallurgical grade silicon (i.e.,a polycrystalline silicon), fumed silica, fluorosilicates and mixturesthereof Some amount of silicon impurities may be present in thefluoroaluminate feed (e.g., as when the fluoroaluminate feed is aby-product of silane production).

Reaction Conditions

Generally, the reactions of the present disclosure occur upon contactingthe fluoroaluminate feed with the acid feed in a reaction vessel so asto suitably form a reaction mixture. The reactions may occur in anaqueous or anhydrous environment as more fully described below.

The molar ratio of acid to fluoroaluminates added to the reaction vesselmay be about the stoichiometric ratio which is dependent on thefluoroaluminate starting material and which may be determined fromreactions i to x (e.g., 5 moles of acid added per mole of chiolite as inreaction iii). Alternatively, a molar excess of acid may be used (e.g.,at least about a 5% molar excess, at least about a 10%, at least about a25%, at least about a 50%, at least about a 100%, at least about a 250%or even at least about a 500% molar excess of acid). In variousembodiments (and depending on the fluoroaluminate starting materialsused), the molar ratio of acid (e.g., HCl or sulfuric acid) fed to thereaction vessel to the amount of fluoroaluminates fed to the reactionvessel (or the ratio of the rates of addition as in a continuous system)may be at least about 1:1, at least about 2:1, at least about 3:1, atleast about 10:1, at least about 25:1, at least about 50:1 or even atleast about 100:1. In some embodiments, the ratio is from about 1:1 toabout 100:1, from about 1:1 to about 50:1 or from about 1:1 to about25:1.

The source of silicon (e.g., sand) may be added to the reaction vesselin a ratio with respect to the fluoroaluminates that is near thestoichiometric ratio. For instance, as shown above in reactions vi to x,the ratio of silicon atoms to fluorine atoms added to the reactionmixture may be about 1:4. Alternatively, silicon may be added in a molarexcess. For instance, the molar ratio of silicon to fluorine atoms addedto the reaction vessel may be greater than about 1:3.5, greater thanabout 1:3, greater than about 1:2 or even at least about 1:1.Alternatively or additionally, the molar excess of silicon may be atleast about 5%, at least about 10%, at least about 25%, at least about50%, at least about 100%, at least about 250% or even at least about500%. In this regard it should be noted that the source of silicon maybe added in an amount other than as listed above. Silicon may be addedin a ratio less than about stoichiometric such that the reaction productcontains both silicon tetrafluoride and aluminum trifluoride (i.e., thereaction results in silicon tetrafluoride when silicon is present andresults in aluminum trifluoride when silicon is consumed and notpresent). Silicon may be added to the reaction vessel separately or maybe mixed with the fluoroaluminate feed prior to introduction into thereaction vessel.

i. Aqueous Reaction Systems

In certain embodiments, an aqueous solution of acid is used in thereactor system. The acid may be present in a reaction vessel in whichthe fluoroaluminate is fed. The acid may be continually fed to thereaction vessel as in a continuous process or a discreet amount of acidmay be present as in a batch process. The acid may be fed as an aqueoussolution of acid or as a gas that is dissolved into an aqueous solutionpresent in the reactor vessel.

In aqueous reaction systems, the contents of the reaction vessel may bemixed continuously by, for example, mechanical agitation (e.g., impelleror bubbling action). In certain embodiments employing an aqueousreaction system, the temperature of the reaction vessel is at ambient(about 20° C. to about 25° C.) and, alternatively or in addition, thetemperature does not need to be controlled during the reaction, i.e., insome embodiments external heat or cooling is not used. In otherembodiments, the temperature of the reactor is maintained at atemperature of at least about 100° C., at least about 150° C., at leastabout 200° C. from ambient to about 300° C., from ambient to about 250°C. or from about 100° C. to about 250° C. Generally, as theconcentration of acid increases, the temperature at which the reactionvessel should be maintained to complete the reaction decreases.

The design of the reaction vessel in aqueous systems is generally withinthe ability of one of ordinary skill in the art and may be dependent onthe desired production rates, conversions, operating temperatures andthe like. In certain embodiments, the reaction vessel is an agitatedtank and, in other embodiments, is a slurry bubble column as describedon p. 23-49 of Perry's Chemical Engineers' Handbook, 7^(th) Ed. (1997)which is incorporated herein by reference for all relevant andconsistent purposes. The slurry bubble column may operate bycontinuously adding by top or side injection the fluoroaluminatematerial (either as a powder or slurry) into an aqueous reaction mixturewithin the column and bubbling in the acid (e.g., via a sparger). Thereaction slurry may be removed from the bottom of the column.Alternatively, the slurry bubble column may operate in a batch modewherein each stream is added to the reactor from the top or side withthe acidic gas being added by a bottom sparger. The reaction may occurfor a desired residence time and the reaction contents may then beremoved from the reactor.

The pressure of the reaction vessel may be about atmospheric or may bemaintained at a pressure of at least about 5 bar, at least about 10 bar,at least about 15 bar, from about atmospheric to about 20 bar, fromabout atmospheric to about 15 bar or from about atmospheric to about 10bar.

Generally, in batch systems, the reaction is allowed to proceed for atleast about 10 minutes, at least about 30 minutes, at least about 60minutes, at least about 90 minutes, from about 10 minutes to about 120minutes or from about 15 minutes to about 60 minutes. In continuoussystems, the residence time in the reaction vessel may be from about 1minute to about 60 minutes or even from about 5 minutes to about 30minutes.

ii. Anhydrous Reaction Systems

In some embodiments, the acid contacted with the fluoroaluminate is asubstantially anhydrous gas stream. For instance, substantiallyanhydrous acid (e.g., substantially anhydrous HCl or sulfuric acid) maybe fed to a reaction vessel in which the fluoroaluminate and optionallya source of silicon are suspended such as, for example, a fluidized bedreactor.

The design of the reaction vessel in anhydrous systems is generallywithin the ability of one of ordinary skill in the art and is dependenton the desired production rates, conversions, operating temperatures andthe like. The reaction system may be batch, continuous or semi-batchwithout departing from the scope of the present disclosure. Inembodiments wherein a fluidized bed reactor is used as the reactionvessel, the fluidized bed reactor may generally be a cylindricalvertical vessel; however, any configuration that is acceptable tofluidized bed operations may be utilized. The particular dimensions ofthe vessel will primarily depend upon system design factors that mayvary from system to system such as the desired system output, heattransfer efficiencies and system fluid dynamics, without departing fromthe scope of the present disclosure.

During operation of the reaction system, the fluidizing gas velocitythrough the reaction zone of the fluidized bed reactor is maintainedabove the minimum fluidization velocity of the fluoroaluminate andoptionally the source of silicon. The gas velocity through the fluidizedbed reactor is generally maintained at a velocity of from about one toabout eight times the minimum fluidization velocity necessary tofluidize the particles within the fluidized bed. In some embodiments,the gas velocity is from about two to about five times and may even beabout four times the minimum fluidization velocity necessary to fluidizethe particles within the fluidized bed. The minimum fluidizationvelocity varies depending on the properties of the gas and particlesinvolved. The minimum fluidization velocity may be determined byconventional means (see p. 17-4 of Perry's Chemical Engineers' Handbook,7th. Ed., incorporated herein by reference for all relevant andconsistent purposes). Although the present disclosure is not limited tospecific minimum fluidization velocities, minimum fluidizationvelocities useful in the present disclosure range from about 0.7 cm/secto about 350 cm/sec or even from about 6 cm/sec to about 150 cm/sec.

Gas velocities higher than the minimum fluidization flow rate are oftendesired to achieve higher productivities. As the gas velocity increasesbeyond the minimum fluidization velocity, the excess gas forms bubbles,increasing the bed voidage. The bed can be viewed to consist of bubblesand “emulsion” containing gas in contact with silicon particles. Thequality of the emulsion is quite similar to the quality of the bed atthe minimum fluidization condition. The local voidage in the emulsion isclose to the minimum fluidization bed voidage. Hence, bubbles aregenerated by the gas introduced in excess of what is required to achievethe minimum fluidization. As the ratio of actual gas velocity to theminimum fluidization velocity increases, the bubble formationintensifies. At a very high ratio, large slugs of gas are formed in thebed. As the bed voidage increases with the total gas flow rate, thecontact between solids and gases becomes less effective. For a givenvolume of the bed, the surface area of solids in contact with reactinggases decreases with increasing bed voidage resulting in reducedconversion to the fluoride product. Accordingly, the gas velocity shouldbe controlled to maintain conversion within acceptable levels.

The temperature of the reaction vessel (including embodiments wherein areaction vessel other than a fluidized bed reactor is used) may bemaintained at a temperature of at least about 75° C., at least about150° C., at least about 200° C., from about 75° C. to about 300° C. orfrom about 75° C. to about 200° C. The heat that is used to maintain thereaction zone at such temperatures may be provided by conventionalheating systems such as electrical resistance heaters disposed on theexterior of the reactor vessel wall. The reaction vessel may operate atpressures from about 1 bar to about 20 bar or from about 1 bar to about10 bar. The residence time in the reactor may be less than about 10minutes, less than about 5 minutes or even less than about 1 minute.

Generally, in both aqueous and anhydrous systems for producing thefluoride product, conversion of the fluoroaluminate to the fluorideproduct may be at least about 50%, and, in other embodiments, at leastabout 60%, at least about 75%, at least about 90%, or even at leastabout 95% (e.g., from about 50% to about 98%, from about 60% to about98% or from about 75% to about 98%).

It should be noted that any reactor capable of carrying out the abovedescribed reactions may be used without departing from the scope of thepresent disclosure. Furthermore, the process of embodiments of thepresent disclosure may be conducted in continuous or batch systems andmay be carried out in a single reaction vessel or may incorporate one ormore reaction vessels configured in series or in parallel.

Recovery of Fluoride Product and By-Product Treatment

The methods of the present disclosure generally involve preparation of afluoride product (e.g., aluminum trifluoride and/or silicontetrafluoride) with one or more by-products. The various products andby-products of the reactions described above are illustrated in Table 1and described more fully below. The equipment and methods to separateand purify the fluoride product (e.g., aluminum trifluoride or silicontetrafluoride) may generally be selected from any of the equipment andmethods known and available to one of ordinary skill in the art withoutlimitation. Anhydrous systems are generally simpler to operate thanaqueous systems as anhydrous systems do not involve slurry treatmentoperations; however anhydrous systems may involve a controlled particlesize distribution of the fluoroaluminate feed (and source of silicon ifany) and may involve higher processing temperatures.

TABLE 1 Products and By-products Produced in Anhydrous and AqueousSystems and in the Presence and Absence of Silicon Aqueous; Aqueous;Anhydrous; Anhydrous; No Silicon Silicon No Silicon Silicon PresentPresent Present Present Fluoride Product AlF₃ SiF₄ (g) AlF₃ (s) SiF₄ (g)(slurried) Solid or Slurried Salt Salt Salt Salt By-products Liquid By-HF HF — — products (Dissolved) (Dissolved) Salt Salt (Dissolved)(Dissolved) Gaseous By- HF HF HF HF products H₂ F₃SiOSiF₃ H₂

In aqueous systems that do not contain a source of silicon, aftercompletion of the reaction, the reaction mixture contains an amount ofaluminum trifluoride product that is slurried in the reaction mixture. Asalt of the acid (e.g., alkali or alkaline earth-metal chloride orsulfate) is typically also present as a slurried solid and/or isdissolved in the aqueous reaction mixture. The reaction also may producean amount of hydrogen fluoride which may be dissolved in the reactionmixture or may be drawn from the reaction mixture in an effluent gas.This effluent gas may also contain an amount of hydrogen gas andunreacted and vaporized acid.

The liquid reaction mixture containing slurried fluoride product may beintroduced into a solid-liquid separation unit to produce a solidfraction containing the aluminum trifluoride product and a salt of theacid (e.g., chloride and/or sulfate salt) and a liquid fractioncontaining hydrogen fluoride, a salt of the acid and an amount ofunreacted acid. Solid-liquid separation units are generally known in theart and include, for example, centrifuges, decanters, filters (e.g.,sieve screens) and the like.

To separate the solid aluminum trifluoride product from the salt, thesolid fraction may be introduced into one or more wash units. Generally,the salt is more soluble in water than the aluminum trifluoride product.The wash unit generally operates by contacting the fluoride/salt solidfraction with water for a sufficient amount of time to allow the salt todissolve into the aqueous phase. The salt-enriched water may then beseparated from the slurried aluminum trifluoride product by a secondsolid-liquid separation unit for product recovery. This secondsolid-liquid separation unit may form part of the wash unit itself. Anumber of wash units may be used and the wash units may be arranged inseries or parallel without limitation. The spent wash water may beprocessed (e.g., by drying such as flash drying) to recover the saltwhich may be sold commercially or further processed as described below.

Aluminum trifluoride product may be dried to remove any remaining waterby the addition of extraneous heat and/or reduction in pressure toremove additional water and/or acid from the product. Suitable dryingtemperatures are at least about 50° C., at least about 100° C., at leastabout 130° C., from about 50° C. to about 150° C. or from about 100° C.to about 150° C.

In this regard it should be noted that when aluminum trifluoride isproduced as a fluoride product, the aluminum trifluoride may be presentin a number of hydrated forms. Without being bound to any particularlytheory, it is believed that aluminum trifluoride solid (e.g., filtercake) that is dewatered in the solid-liquid separation device is in thetrihydrate form, AlF₃•3H₂O. Further it is believed that drying resultsin dehydration of the product and formation of at least one of themono-hydrate, semi-hydrate or even anhydrous form of aluminumtrifluoride.

The liquid fraction separated from the solid fraction in thesolid-liquid separation device and the effluent gas removed from thereaction vessel may be introduced into a distillation column to removeand separate one or more of the unused acid, hydrogen fluoride andhydrogen gas. The design and operation of distillation methods aregenerally within the skill of one of ordinary skill in the art and aredependent on various factors including the composition of the feed, thedesired recovered product(s), the desired recovery and the like.Unreacted acid may be recycled back to the reaction vessel in continuoussystems.

In anhydrous systems in which silicon is not present such as, forexample, a fluidized bed operation in which an anhydrous acid gas isbubbled through a fluidized bed of fluoroaluminate material, thereaction produces solid aluminum trifluoride product which may bewithdrawn from the reactor. The product particulate may include anamount of solid by-product salt (e.g., NaCl, NaHSO₄ or Na₂SO₄) which maybe separated out as described below. Hydrogen fluoride and hydrogen gasmay be generated as gaseous by-products that are withdrawn from thereaction vessel with unreacted acid.

In such anhydrous systems in which silicon is not present, theparticulates that typically include aluminum trifluoride product andsalt may be introduced into one or more wash units to separate the saltfrom the aluminum trifluoride product. The wash units may be similar tothe wash units described above for aqueous systems. After washing, thesolid product may be dried to at least partially dehydrate the fluorideproduct as described above. The spent gas that is removed from thereaction vessel may be subjected to distillation to recover at least oneof unreacted acid, hydrogen fluoride and hydrogen gas.

In both aqueous and anhydrous systems, when silicon is present in thereaction vessel and is available for reaction, silicon tetrafluoride gasis produced as a product. In aqueous systems, a salt of the acid may beslurried within the reaction mixture as a by-product. In such aqueoussystems, the reaction also may produce an amount of hydrogen fluoridewhich may be dissolved in the reaction mixture and/or may be withdrawnfrom the reaction mixture with the product gas. This product gas mayalso contain an amount of vaporized acid and/or F₃SiOSiF₃ by-product.

In anhydrous systems that contain a source of silicon, the solidfluoroaluminate decomposes into particulate salt (e.g., NaCl, NaHSO₄ orNa₂SO₄) during the reaction. Hydrogen fluoride may be generated as agaseous by-product that is withdrawn from the reaction vessel with anyunreacted acid and silicon tetrafluoride. In both aqueous and anhydroussystems that produce silicon tetrafluoride product gas, the silicontetrafluoride gas may be separated from the other gases by distillation,acid baths (e.g., sulfuric acid bath to remove unreacted HF) and/oradsorption units (e.g., a zinc-based adsorber to remove acid) which maybe operated in any combination and number and may be operated in seriesor parallel without limitation. Silicon tetrafluoride product gas may becondensed for storage as a liquid product and/or may be furtherprocessed by, for example, reaction with an alkali or alkalineearth-metal aluminum tetrahydride for the production of silane.

In certain embodiments and regardless of whether aqueous or anhydrousacid is used and regardless of whether the reaction occurs in thepresence of silicon, hydrogen fluoride by-product may suitably bereacted with a source of silicon to produce silicon tetrafluoride gas.Hydrogen fluoride may be separated from other gases in a distillationcolumn. In this regard it should be noted that it is not necessary toremove the unreacted acid from the hydrogen fluoride as the acid doesnot interfere with production of silicon tetrafluoride. The hydrogenfluoride may be introduced into a reaction vessel in which a source ofsilicon (e.g., sand) is present such as a packed bed or fluidized bed toproduce silicon tetrafluoride. The silicon tetrafluoride gas may bewashed with sulfuric acid to remove further by-product gases and may beintroduced into an adsorber, preferably with zinc media, to remove anyunreacted acid.

Dissolved chloride or sulfate salts (e.g., present in the reactionsolution and/or dissolved during washing operations) may be recovered bydrying. Such drying operations typically vaporize any unreacted acidpresent in the solution which allows the acids to be recovered forre-use. Recovered by-product chloride or sulfate salts may becommercially sold or may be reacted with fluorosilicic acid toregenerate the starting acids (HCl or sulfuric acid) and producefluorosilicates which may be used as starting materials for theproduction of the fluoride products of the present disclosure (e.g.,silicon tetrafluoride). For instance, the fluorosilicates may be used asthe source of silicon to produce silicon tetrafluoride.

Production of Silane and Fluoride Product

The fluoride production methods described above may generally beincorporated into a process for producing silane such that theby-products of silane production may be used to generate value-addedproducts. In one or more exemplary embodiments, silicon tetrafluoride iscontacted with an alkali or alkaline earth-metal salt of aluminumtetrahydride to produce silane and an effluent that contains one or morefluoroaluminates. As described above, the fluoroaluminate may becontacted with an acid to produce aluminum trifluoride (in the absenceof silicon) or silicon tetrafluoride (in the presence of silicon) and atleast one by-product which may be separated from the fluoride product.

Silicon tetrafluoride starting material may be produced by evaporatingsolutions of fluorosilicic acid. Alternatively or in addition, a portionof the silicon tetrafluoride that is reacted with aluminum tetrahydrideto produce silane may be generated from the methods described above.Alkali or alkaline earth-metal salts of aluminum tetrahydride may beproduced by reacting their elemental precursors (Na, Al and H) underhigh pressure and temperature.

Production of silane is generally described in U.S. Pat. No. 4,632,816which is incorporated herein by reference for all relevant andconsistent purposes. Gaseous silicon tetrafluoride may be introducedinto an agitated liquid reaction medium containing aluminum tetrahydridesalt. The liquid reaction medium may include solvents selected frompolyethers (e.g., diglyme, monoglyme or dioxane), hydrocarbons (e.g.,toluene or pentane) and mixtures thereof. The reaction mixture may bemaintained from about 30° C. to about 80° C. and atmospheric pressuremay be used. The reaction mixture may also be maintained at higherpressures such as pressures up to about 100 atm. In some embodiments,the reaction medium is maintained at a pressure of from about 1 to about10 atm.

Stoichiometric amounts of silicon tetrafluoride and aluminumtetrahydride may be used to produce silane; however, in some embodimentsa molar excess of tetrahydrides is used to suppress formation ofby-products. The reaction may be performed batch-wise or continuouslysuch as in a continuous back-mixed reactor or in a slurry bubble column.

The reaction generates silane gas and slurried fluoroaluminate salt. Thefluoroaluminates may be separated from the reaction medium by meansgenerally known in the art such as by use of solid-liquid separationunits (centrifuges, decanters, filters and the like). Upon separation,the fluoroaluminates may be introduced to a reaction vessel with acid toa produce the fluoride product (aluminum trifluoride or silicontetrafluoride) as described above.

EXAMPLES Example 1 Production of Aluminum Trifluoride by HydrochloricAcid Digestion of Fluoroaluminates with Continuous Exhaustion ofGenerated Gas

A solid mixture (15.7 g) of sodium aluminum fluoride (NaAlF₄), chiolite(Na₅Al₃F₁₄) and cryolite (Na₃AlF₆) (“fluoroaluminate mixture”) was mixedwith silica (8 g). The solids mixture was then mixed in a TEFLON beakercontaining aqueous hydrochloric acid (243 g at 36 wt %). The initialmole ratio of hydrochloric acid to the fluoroaluminate mixture was 20:1.A magnetic stirrer was placed at the bottom of the beaker for mechanicalagitation of the mixture. The beaker was at an ambient pressure of 1 barand at an ambient temperature of 20° C. The fluoroaluminate powderreacted violently with aqueous hydrochloric acid to produce fumes (SiF₄)that were exhausted continuously. The mixture was agitated for 45minutes when the grayish slurry of the fluoroaluminate mixture andaqueous hydrochloric acid completely turned to a whitish slurry. Theliquid in the slurry was decanted and the resulting solids mixture wasdried under a lamp to yield 27.3 g of solids. Analysis of dry solidsindicated the loss in the fluorine moles to be 11%, which on weightbasis was equivalent to the gain in the chlorine moles. Based onstoichiometry, the conversion from fluoroaluminates to aluminumtrifluoride semi-hydrate and hydrogen fluoride was estimated to beapproximately 60%.

Example 2 Production of Aluminum Trifluoride by Hydrochloric AcidDigestion of Fluoroaluminates in an Enclosed Vessel

A fluoroaluminate mixture (24.7 g) was mixed with 36 wt % hydrochloricacid in an enclosed digestion vessel made of TEFLON. The vessel and thecontents were heated to 150° C. and the relief valve on the vessel wasset to release at 100 psig. After 30 minutes of heating, the contents ofthe vessel were cooled to ambient and the relief valve was opened. Theloss in the weight of the vessel or the gas released was 0.11 g. Theliquid in the digestion vessel was decanted and the solids mixture wasdried under a lamp. The solids yield on drying was 28%. The resultingsolids were washed with water and dried again. The yield of solids onthe second drying was 64%. Based on stoichiometry, the conversion offluoroaluminate to aluminum trifluoride semi-hydrate was 93%.

When introducing elements of the present disclosure or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

As various changes could be made in the above apparatus and methodswithout departing from the scope of the disclosure, it is intended thatall matter contained in the above description and shown in theaccompanying figures shall be interpreted as illustrative and not in alimiting sense.

1. A method for producing aluminum trifluoride, the method comprising:contacting a fluoride salt of alkali metal or alkaline earth-metal andaluminum with an acid in the absence of a source of silicon to producealuminum trifluoride and at least one by-product, the acid beingselected from hydrochloric acid, sulfuric acid and mixtures thereof; andseparating the aluminum trifluoride from the by-product.
 2. The methodas set forth in claim 1 wherein the fluoroaluminate feed comprises atleast about 70% by weight fluoride salts of alkali metal or alkalineearth-metal and aluminum.
 3. The method as set forth in claim 1 whereinthe fluoride salt of alkali metal or alkaline earth-metal and aluminumis selected from the group consisting of NaAlF₄, Na₅Al₃F₁₄, Na₃AlF₆ andmixtures thereof.
 4. The method as set forth in claim 1 wherein hydrogenfluoride and a chloride or sulfate salt of alkali or alkalineearth-metal are produced as by-products.
 5. The method as set forth inclaim 4 wherein the hydrogen fluoride is contacted with a source ofsilicon to produce silicon tetrafluoride.
 6. The method as set forth inclaim 1 wherein the fluoride salt is a by-product of silane production.7. The method as set forth in claim 1 wherein the contacting stepcomprises contacting the fluoride salt of alkali metal or alkalineearth-metal and aluminum with the acid that is in aqueous form in areaction vessel to produce a slurry containing the aluminum trifluorideand the at least one by-product.
 8. The method as set forth in claim 7wherein the slurry is introduced into a solid-liquid separation unit toproduce a solid fraction containing the aluminum trifluoride and aliquid fraction containing at least one by-product.
 9. The method as setforth in claim 8 wherein the solid fraction contains aluminumtrifluoride and a chloride or sulfate salt of an alkali metal oralkaline earth-metal and the liquid fraction contains water, hydrogenfluoride, unreacted acid and a chloride or sulfate salt of an alkalimetal or alkaline earth-metal.
 10. The method as set forth in claim 9comprising introducing the solid fraction into a wash unit to separatethe aluminum trifluoride from the chloride or sulfate salt.
 11. Themethod as set forth in claim 9 comprising separating at least one ofwater, hydrogen fluoride and unreacted acid in a distillation column.12. The method as set forth in claim 1 wherein the contacting stepcomprises contacting the fluoride salt of alkali metal or alkalineearth-metal and aluminum with the acid that is substantially anhydrousand the fluoride salt is introduced to a fluidized bed reactor whichcontains the acid as a fluidizing gas.
 13. The method as set forth inclaim 12 wherein particulate aluminum trifluoride and a chloride orsulfate salt of alkali or alkaline earth-metal are produced in thefluidized bed reactor.
 14. The method as set forth in claim 13comprising introducing the particulate aluminum trifluoride and thechloride or sulfate salt into a wash unit to separate the aluminumtrifluoride from the salt.
 15. The method as set forth in claim 13wherein a spent gas is produced, the spent gas comprising hydrogen,hydrogen fluoride and unreacted acid.
 16. The method as set forth inclaim 15 comprising separating at least one of hydrogen, hydrogenfluoride and unreacted acid from the spent gas in a distillation column.17. The method as set forth in claim 1 wherein aluminum trifluoridehydrate is produced.
 18. The method as set forth in claim 17 wherein thealuminum trifluoride hydrate is dried to form aluminum trifluoridesemi-hydrate.
 19. The method as set forth in claim 1 wherein thefluoride salt is a particulate with an average nominal diameter of lessthan about 500 μm.